It is both a function and a option. The functions are called NthLargest and NthSmallest. The option is under the Report menu item and is called Group Sort Expert. Unfortunately I do not think this will work for what you are trying to achieve.
Lockwelle suggested doing a counter. I think if you were grouped by donor, sorted by date (descending), suppress when the counter >10, reset counter at the group level.
You also might be able to do a group selection. I will have to try a test.